Caloric nystagmus after plugging of the horizontal semicircular canal in pigeons

The mechanism of caloric nystagmus has been attributed to endolymphatic thermoconvection flow of the horizontal semicircular canal in the inner ear. It has recently become difficult, however, to explain caloric nystagmus provoked in the space experiment by employing the convection theory only because the convection phenomenon does not exist under the conditions of microgravity (0G). Although several hypotheses concerning the mechanism of caloric nystagmus have been proposed which exclude the convection theory, opinions remain divided among investigators as to the mechanism governing caloric nystagmus under conditions of normal gravity (1G) and 0G. In this study, we confirmed that the convection theory represents the mechanism of caloric nystagmus under 1G using pigeons (Columba livia). The direction of caloric nystagmus was dependent upon the head position and the stimulus site of calorization in the semicircular canal. But after the plugging of the semicircular canal, however, caloric nystagmus disappeared. These results strongly support the theory that the primary mechanisms of caloric nystagmus under 1G are controlled by the convection phenomenon of the endolymph in the semicircular canal.